This invention relates generally to amplifiers and, more particularly, to a low noise, high frequency operational amplifier.
Prior art low noise operational amplifiers such as the OP-27 manufactured by Precision Monolithics, Inc. utilize an NPN differential input stage including load resistors as the collector load impedances. These load resistors are chosen to be large so as to provide a high gain input stage and reduce offset voltage and noise. That is, the offset voltage and noise associated with the second stage is divided by the gain of the first stage.
The OP-27 operational amplifier also represents an attempt to achieve high frequency performance. While the device exhibits satisfactory phase margin (i.e. excess phase at unity gain), the gain characteristic of the device is such that after it passes down through the unity gain frequency, the gain rises again at a higher frequency. If the device has gain at the point where the phase approaches zero, the circuit will oscillate. This tendency to oscillate at higher frequencies in the unity gain configuration renders the frequency response of the device undesirable.
One problem faced by designers of operational amplifiers is that it is generally necessary to provide level shifting circuitry between the amplifier's input and output. When using an NPN input stage as above described, lateral PNP transistors are generally utilized to accomplish the level shift if the resulting operational amplifier is to have large dynamic supply voltage range. Unfortunately, lateral PNP transistors do not exhibit a very satisfactory behavior as a function of frequency. In the case of the OP-27 operational amplifier, several capacitive compensation circuits are employed to improve the frequency performance. Typically, as much as 370 pf of capacitance are used which occupies a great deal of space on a semiconductor die.